The second and smaller of the two asteroids, 2010 RF12, is estimated to be 20 to 46 feet across and will pass within about 49,088 miles (79,000 kilometers) of the Earth.

The first asteroid, 2010 RX30, was 32 to 65 feet across.

If either had collided with the Earth, how much of a risk would they pose? Are they big enough to cause serious damage?

It's hard to tell, according to a Universe Today blog on the two asteroids. But author Jon Voisey speculates that "The majority of the mass for such small objects would burn up in the atmosphere with only small fragments surviving to the ground. For comparison, the estimated size of the object that caused the Tunguska event was estimated to be at least a few tens of meters in diameter at the point it exploded in the atmosphere some few miles up."

NASA's Donald Yeomans, manager of the Near Earth Program, told CNN that this kind of near miss is good practice, and indicates room for improvement given that the incoming objects were only spotted Sunday. "This demonstrates the system's working on some level, but we need larger telescopes and more of them to find objects that are coming this close."